1. Field of the Invention
The present invention relates to a pentagonal mirror unit of a single lens reflex camera.
2. Description of Related Art
In a known finder of a single lens reflex camera, an object image (light) transmitted through a taking lens and reflected by a quick return mirror (first mirror) is focused on a focusing plate and is then reflected by upper roof surfaces of a pentagonal prism (pentagonal roof prism) and a third reflecting surface (front surface) thereof. The object image (light) is then made incident upon a magnifying lens (eyepiece) to obtain an erect image.
However, the finder using the pentagonal prism has drawbacks in that the pentagonal prism is expensive and heavy. To eliminate the drawbacks, it is known to use a hollow pentagonal mirror having reflecting mirrors corresponding to the roof surfaces and the third reflecting surface, in place of the conventional solid pentagonal prism. The hollow pentagonal mirror is lighter and less expensive than the solid pentagonal prism, since the glass body portion of the solid pentagonal prism is replaced with a cavity.
The hollow pentagonal mirror is usually secured to the camera body through a mirror sheet. In a conventional apparatus, the hollow pentagonal mirror is provided with an elastically deformable leg having an engaging pawl outwardly projecting at the free end of the elastically deformable leg. The mirror sheet can be provided with an engaging hole in which the engaging pawl of the elastically deformable leg is engaged to secure the hollow pentagonal mirror to the mirror sheet. Upon securing the pentagonal mirror to the mirror sheet, the elastically deformable leg of the pentagonal mirror is elastically deformed to be inserted in the mirror sheet to engage the engaging pawl in the corresponding engaging hole.
However, in the conventional apparatus, the elastic deformation of the elastically deformable leg causes the side surface of the pentagonal mirror and the roof surfaces thereof to deform which results in an adverse influence on the optical efficiency of the assembled pentagonal mirror. Furthermore, since the elastically deformable leg and the engaging pawl extend downwardly from the lower end of the pentagonal mirror, the injection molding dies, which are used to make the pentagonal mirror, are complicated and expensive.